Direct current motors, such as the ones used as pump drives for hydraulic pumps in motor vehicle braking systems, are frequently controlled in a PWM operation (PWM: pulse width modulation), in which the direct current motor is periodically switched on and off, using a prespecified pulse control factor. By the suitable selection of the pulse duty factor, the rotary speed of the motor is able to be set and regulated as desired. The pulse duty factor (defined by the “on” time/period duration) in this context determines the rotary speed of the direct current motor, and may basically be selected between 0% (completely switched off) and 100% (durably switched on).
FIG. 1 shows a typical system for rotary speed regulation of a direct current motor 1. The system includes direct current motor 1 that is to be regulated, an electronic system 2, connected to it, having a switching output stage and a control unit 3 connected to electronic system 2. The terminal voltage present at direct current motor 1 is designated by Uk, and the current flowing through the motor is designated by Imot.
Control unit 3 includes a control algorithm 7, which generates a PWM signal 6 as a function of the system deviation (see FIG. 2), using which the switching output stage of electronic system 2 is periodically switched on and off. Controller 7 usually operates at a clock-pulse rate that is higher than the clock-pulse rate of PWM signal 6.
FIG. 2 shows a typical PWM signal 6 for controlling a direct current motor 1. During power up phases 4, the switch of the switching output stage is closed, and during turnoff phase 5 it is open. Thereby, corresponding to the pulse duty factor, direct current motor 1 is periodically connected to supply voltage Ubatt or disconnected from it. The duration of the individual phases is denoted by ton and toff, in this context. The pulse duty factor is given by: V=ton/T.
The current actual rotary speed ω of motor 1, which flows into control algorithm 7 as an input variable, is usually calculated from the so-called tracking voltage. In this context, the tracking voltage is a regenerative terminal voltage Uk of direct current motor 1, which is measured in a switch-on phase 5 of the PWM signal. For rotary speed ω, ω=f(Uk) applies. The determination of the rotary speed from the tracking voltage of a pump motor is known, for example, from German Patent Application No. DE 199 14 404.
The instantaneous rotary speed ω can only be measured during turnoff phases 5 (indirectly), in which motor 1 generates a regenerative voltage Uk. During power up phase 4, on the other hand, no rotary speed measurement is possible. Since control algorithm 7, as a rule, operates at a higher clock-pulse rate and also requires rotary speed values during power up phases 4, the rotary speed is estimated in these phases 4. In order to do this, an average rotary speed is calculated, for instance, in turn off phase 5, and this rotary speed is assumed to be valid also in power up phase 4. However, known estimation methods are relatively inaccurate, since motor 1 is greatly accelerated during power up phase 4, and is braked by the load and mechanical friction during turnoff phases 5. The accuracy of the rotary speed regulation is considerably impaired thereby.
An additional problem of usual PWM operation comes about from the relatively low frequencies of the PWM signals used, of, for instance, 50 Hz. Because of the long power up and turnoff phases 4, 5, rotary speed ω of electric motor 1 fluctuates relatively greatly about the setpoint value. Particularly in the case of small pulse duty factors such as 20%, the rotary speed fluctuations are very strong because of the short power up phases 4 and the relatively long turnoff phases 5. This, in turn, has negative effects upon assemblies that are driven by direct current motor 1.